The present invention broadly relates to integrated optoelectronic photodetector circuits and to fabrication techniques therefor. More particularly, the invention concerns methods for fabricating arrays of photonic devices consisting of alternating light-emitting diodes (LEDs) and PIN photodetectors operable at the same wavelength or alternating LEDs that are operable at two different wavelengths. One-step epitaxial growth of all layers for the active devices and simple planar processing assure high-yield, reproducible fabrication of devices having uniform characteristics.
The development of integrated optical circuits has become the subject of increased interest, especially in the area of fiber optic communications. A number of difficulties have been encountered in these efforts to fabricate different types of active devices next to each other on the same wafer.
Previously employed techniques have utilized selective epitaxial growth and multiple growth runs to integrate LED and PIN structure having different layers of III-V compound materials. Selective epitaxy on the same substrate is very difficult when the LED and PIN spacing in the arrangements is very small. Because of this, an alternative approach has been to fabricate III-V LED layers on a silicon substrate and thereafter use the silicon substrate to form the photodetectors. The problem with this approach is that the large lattice mismatch between the III-V epitaxial layers and the silicon substrate will degrade the LED quality and limit the detector's response to 0.8 micrometers (.mu.m) wavelength.
A further problem has been the difficulty of optimizing each of the separate components on the wafer. As a result, it has been heretofore necessary to compromise the performance of one or more the components. Where the circuits include PINs, it is usually these components that are compromised.
Nonoptimized PINs may be acceptable for monitoring applications or for use in generating electrical signals for switching, clocking and triggering. However, in a fiber optic communication system the speed and sensitivity of the detector are far more crucial than in these applications. The requirements are especially stringent for integrated emitter/detector systems which operate at 1.3 .mu.m wavelengths. For these applications, the detector must have low noise so that the system can be operated at a low bit error rate (BER), e.g., BER=10.sup.-9. The detector must also have a high sensitivity so that it can detect signals having powers less than a microwatt. As well, the detectors must have this sensitivity over a high bandwidth so that they can operate at data rates greater than 200 Mbits per second. To satisfy all of these requirements, the detector must have a dark current (I.sub.d)&lt;10 nA (at -10 volts), quantum efficiency (QE)&gt;75% and a capacitance &lt;1 pF (at -10 volts).